Why do we burn carbohydrates over fats?

Hi all. Does anyone have an evolutionary and/or biochemical reason for why carbs are burned before fats when we eat them together? My brother was using this as a reason for why we should eat lots of carbs, and I couldn't give him a good answer.

Sugar becomes toxic after too much time spent in the blood. There's a reason why insulin spikes in healthy people are around 3 hours. It starts becoming a toxin after that.

IMO, my common sense tells me that the body burns energy in order or toxicity.

Alcohol is the first macronutrient to be burnt. It's highly toxic and is quickly broken down into acetates for energy. As it is poison, all other metabolic processes are stopped until it's out of the system.

Carbohydrate is toxic when elevated in the blood as sugar for too long. Carbohydrate stores in the body are also the most easily depleted and refilled, plus they're rather small. They're like nitrous tanks on a race car - not necessary to go fast yet they're a big turbo boost, but they'll start breaking stuff if they're used all the time (hey, that's a pretty good analogy I just came up with!) They're not acute poison like alcohol is, but they can be problematic in large quantities.

Fats are next on the list. They are largely harmless (minus trans fats and too many polyunsaturates). Since they're the most stable, it is therefore in the body's best interest to burn these as fuel last. The body is okay with just letting this type of energy just sit around, so it's not going to use it unless the other more toxic fuels are removed first.

Protein is actually harmful to use as fuel. Your body deteriorates when protein is used, so logically this is the last choice.

At least this is how it goes in my head.

Don't put your trust in anyone on this forum, including me. You are the key to your own success.

I agree with what Choco says, but would like to add that the order of fuel 'preference' is due to the body's constant function of maintaining and trying to maintain homeostasis. Homeostasis is basically the natural at rest state your body wants to be in. When you eat carbohydrates you get the following insulin spike because your body is actively trying to return to low levels of blood sugar. By constantly eating carbohydrates your body is constantly trying to return to low levels of blood sugar, and struggles over time to return to homeostasis. Just like alcohol, which Choco mentioned, you're body tries to processes the alcohol as fast as it can in order to return to normal fuel sources. The most normal fuel source being body fat.

Since your body prioritizes alcohol even over carbs, by your brothers logic he shouldn't be loading up on carbs he should be loading up on alcohol. But we all know how that would turn out.

I did mention the alcohol analogy, and he said 'but alcohol is obviously poisonous', to which I replied that carbs were toxic in high amounts, so it looks like we're all thinking along the same lines! The homeostasis thing makes sense, thanks!

I think the biochemical reason is simply that when you have lots of glucose in the blood, your insulin is raised and insulin prevents fat from leaving the fat cells at the same time it allows fat to move into the fat cells. As long as insulin is present in a high enough level, you are prevented from using fat for energy. It's not so much a preference as just the way the body works. Without the raised insulin your body prefers to use other sources of energy.

From what I have read, rabbit starvation isn't because of too much protein. It is because it can't be burned fast enough. Rabbit starvation occurs because rabbits don't have enough fat or carb along with lots of protein. It may also be that we burn carbs so quickly because we are adapted to eating and using carbs, and getting good energy supplies from fat is more rare.

Your body can only store so many carbs in the form of glycogen in the muscles and liver, but it can store an indefinite amount of fat. So your body preferentially burns carbs for energy and stores fat for 'later use'. The problem is that in modern times 'later use' (lean years) never happens.

If we're consuming a mixed diet carbs will be dealt with first. I don't see glucose as toxic because insulin is released to remove it from the blood. There is just a preferred amount that is kept in the blood for homeostasis. Insulin is doing its required job of allowing glucose to be taken up and used for energy or stored as glycogen.

Wikipedia:
The actual amount of glucose in the blood and body fluids is very small. In a healthy adult male of 75 kg with a blood volume of 5 litres, a blood glucose level of 5.5 mmol/L (100 mg/dL) amounts to 5 grams, slightly less than two typical American restaurant sugar packets for coffee or tea.[6] Part of the reason why this amount is so small is that, to maintain an influx of glucose into cells, enzymes modify glucose by adding phosphate or other groups to it.

That means a 12-oz Coke with 39g of sugar has nearly 8 times the amount of sugar in it that your blood should have throughout your entire body.

Glucose Toxicity
Whatever the reason for the failing first phase insulin release there's an ugly feedback mechanism that kicks in when blood sugar levels rise because of that failing first phase insulin release: High levels of circulating glucose themselves are toxic to beta-cells, a phenomenon called "glucose toxicity". So as blood sugars rise these high blood sugar concentrations further damage and or kill more beta-cells, making first and second phase insulin release even less able to control blood sugar concentrations.

Beta cells are in the pancreas, and secrete insulin when needed.

From the same page (I love the paragraph title):

The Fasting Blood Sugar Death Spiral

When the beta-cells are no longer to keep fasting blood sugar normal, this is often a sign that the pancreas no longer has enough beta cell capacity to keep up even with the production of even low levels of insulin needed for basal insulin secretion. This usually signals that a critical amount of irreversible beta-cell death has occurred.

Sugar in your bloodstream will kill you in high quantities. Fats won't. Therefore, your body will process the sugar first, to avoid killing itself.